Use UNSPECIFIED status when logging issue-only status with no issue.

This is clearer and more correct than UNKNOWN.
We could also use INFORMATION for this case, but using UNSPECIFIED seems
nicer as it allows distinguishing between the "no issue" and "issue, but
green" cases.

Bug: 264999473
Test: atest CtsSafetyCenterTestCases
Relnote: Source status logging fix
Change-Id: I83ab02333043f6f01a3466fff8ff924bb36bfb2d